But this week, that meme was thrust tragically into the spotlight, as reports surfaced that two 12-year-olds who authorities say lured a friend into the woods and stabbed her more than a dozen times may have been inspired to commit the brutal crime by the modern-day ghost story.

The alleged attackers, who have been charged as adults, had encountered the tale of Slender Man online and "wanted to prove the skeptics wrong," according to details of a criminal complaint reported by The Associated Press. Possibly under the belief that they had to "physically kill someone" to become a "proxy of Slender," the girls plotted for months to kill their friend, according to the complaint. The victim survived the attack and is in the hospital in stable condition.

Slender Man first surfaced in 2009, when a user on an online forum called Something Awful posted a doctored photo and fictional article in response to a call for fake supernatural images, according to the site KnowYourMeme.com and an expert who has studied the evolution of the Slender Man myth. While such creations aren't uncommon, Slender Man struck a nerve. Soon, users were contributing their own visions of the monster and versions of the terror he'd inflict on unsuspecting school children and others.

“That particular forum just went wild with it," said Shira Chess, an assistant professor at the University of Georgia's Grady College of Journalism and Mass Communication who has written about Slender Man. "They thought it was a compelling villain and so everybody just jumped on and started defining it in different ways."

In variations of the legend, Slender Man was said to "cause memory loss, insomnia, paranoia, coughing fits," according to Know Your Meme. His narrow frame was accented by tentacles or arms that could stretch and retract in some accounts.

Eric Knudsen, the man who takes credit for creating Slender Man and has reportedly since copyrighted the concept, said in an interview earlier this year that he sees the character as a creature whose "body can morph."

"I like the concept of a monster, a creature that causes general unease and terror," he said on a podcast for the program "On the Media." "Its methods are strange, its motives are completely inscrutable. So what is it doing? Why is it here? Is it taking people?"

The story spread, inspiring a spin off online video series, video games and threads on various message boards and sites dedicated to scary folklore. It has since become a popular topic on the site Creepypasta, where the two girls charged with the attack reportedly told authorities they encountered the tale.

It's not clear what could have made the alleged attackers believe they needed to kill in name of the Slender Man. An attorney for one of the girls charged in the case says his client showed signs of mental illness. Both suspects have reportedly expressed remorse for their actions.

Chess, the assistant professor of mass media arts, said she has not come across any iterations of the Slender Man story that invoked violence on the part of believers. She said the tragic case may highlight the need to teach youth media literacy skills that can help them sort out fact from fiction online.

“It’s been out there for five years. They’re just looking at the fact that they see Slender Man everywhere on the Internet," she said. "If you do a search for Slender Man on the Internet, you’ll see content everywhere and it looks like it exists. There’s no disclaimer on the Internet.”

A self-described administrator of Creepypasta published a lengthy post defending the site and its signature genre, saying "placing blame solely on an interest in reading/writing about horror, paranormal, myths, urban legends, etc. for a tragedy would be off the mark." The site, the user wrote, has tried to keep mentions of Slender Man limited as that story is not its only focus. Creepypasta, the admin added, was created to cater to users in high school and older.

"[For] the really young kids… while I don’t believe that creepy stories will cause them to become evil or sick, I do think it could scare them and/or make them very anxious!" the administrator wrote. "And if your child has issues with violent or destructive or depressive issues, it’s really important to make sure that they’re not interacting with things that will exacerbate that."

Knudsen issued a statement Wednesday saying he is "deeply saddened by the tragedy in Wisconsin and my heart goes out to the families of those affected by this terrible act." In the podcast posted earlier this year, Knudsen said he was surprised to see how far his creation had traveled online. The transformative nature of the Internet, he said, has caused him to feel less like the creator of Slender Man and more like the manager of the essence of the character. Some fans, he said, don't even believe he was the man behind the myth.

"It's kind of a double-edged sword, the fact that as an urban legend, we seek to obscure its own past to make it seem more real. But that also makes it seem like it's something that's always been there, rather than something that's fairly recently created," he said.

"Certain people just don't want to believe that it's fiction," he added, "because it's obviously much scarier that way."